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why are my 18650 cells shorting? Answered

After realizing lead acid batteries are sooo 1859, I decided to construct a battery pack out of 18650 cells. I soldered 4 in series on the top half of the cells. When I went to do the bottom half I found that touching the wire from one negative terminal of one cell to the positive of another cell it short circuited. At first I thought I had a bad cell so I checked all of them and they were all fine. I desoldered them and tried putting only 2 in series, same thing. The cells still put out ~3.7 so I don't see the problem. please help. 


YES - Connecting the pos and Neg will create a circuit between the cells. You need to connect pos to neg - pos to neg. As in the diagram.


thanks everyone i had them connected wrong lol


2 years ago

You should not hand iron solder these batteries as heat risks damage
to the chemistry weather in series or boss (parallel) unless it has
resistance welded tabs. You may solder the end of the tab to another end
of tab.Resistance welds attach flat wire in less then a second and do NOT heat the chemistry see pictures.

WeldsCloseUp_002.jpgseries-tab-end joined.jpgPackBuild30.JPGEnergizerNiMHTabs.jpg

also, when i wire them in parralel (spelled like a boss) it does what its supposed to do (ie: not short circuit)

Are you certain you solder on the right spots and in the right direction?
On the pics there are only 2 cells and if you measure the voltage on the unsoldered ends you should have around 7.4V.
If you add more batteries you have to go from negative of one battery to positive of the next, but that is obvious ;)
The thing some people miss is the other end of the battery pack.
If I take your pic and would solder a wire over the other end too it would smoke.
But add another battery and solder it to the second on the bottom only and you have tripled the voltage.

To confirm the possible error just line them up in a row and measure from one end to the other - if that gives proper results you made a mistake during the soldering ;)


2 years ago

Some of these cells include a micro control PCB under the plate.